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How do galaxies grow?

Composite colour-image of the brightest galaxies in four groups located about 4 billion light-years away. The galaxies are ordered in increasing stellar mass, i.e. a rough time sequence. The brightest galaxies in group 1 and 2 both have obvious bright, gravitationally bound companions. Further images obtained with the Hubble Space Telescope shows that the brightest galaxy in group 3 has a double nucleus. Thus, these galaxies are currently in the process of merging. This discovery provides unique and powerful validation of hierarchical formation as manifested in both galaxy and cluster assembly.